Structural requirements for the activation of the human growth hormone secretagogue receptor by peptide and nonpeptide secretagogues

Antibodies raised against an intracellular and extracellular domain of the GH secretagogue receptor (GHS-R) confirmed that its topological orientation in the lipid bilayer is as predicted for G protein-coupled receptors with seven transmembrane domains. A strategy for mapping the agonist-binding site of the human GHS-R was conceived based on our understanding of ligand binding in biogenic amine and peptide hormone G protein-coupled receptors. Using site-directed mutagenesis and molecular modeling, we classified GHS peptide and nonpeptide agonist binding in the context of its receptor environment. All peptide and nonpeptide ligand classes shared a common binding domain in transmembrane (TM) region 3 of the GHS-R. This finding was based on TM-3 mutation E124Q, which eliminated the counter-ion to the shared basic N+ group of all GHSs and resulted in a nonfunctional receptor. Restoration of function for the E124Q mutant was achieved by a complementary change in the MK-0677 ligand through modification of its amine sidechain to the corresponding alcohol. Contacts in other TM domains [TM-2 (D99N), TM-5 (M213K, S117A), TM-6 (H280F), and extracellular loop 1 (C116A)] of the receptor revealed specificity for the different peptide, benzolactam, and spiroindolane GHSs. GHS-R agonism, therefore, does not require identical disposition of all agonist classes at the ligand-binding site. Our results support the hypothesis that the ligand-binding pocket in the GHS-R is spatially disposed similarly to the well characterized catechol-binding site in the beta(2)-adrenergic receptor.